The production of viscous heavy oil from subsurface reservoirs presents significant challenges. For example, some heavy crude oils, such as bitumen, are highly viscous and therefore immobile at the initial viscosity of the oil at reservoir temperature and pressure. Indeed, such heavy oils may be quite thick and have a consistency similar to that of peanut butter or heavy tars, making their extraction from the reservoir especially challenging.
Conventional approaches to recovering such heavy oils focus on methods for lowering the viscosity of the heavy oil so that the heavy oil may be produced from the reservoir. One example of a conventional method for recovering heavy oil is surface mining. Surface mining may be infeasible or at least highly inefficient under certain circumstances, such as when the desired hydrocarbons are not located near the surface. Additionally, in some of the conventional approaches, surface mining may require significant surface reconstitution.
Another example of a conventional method for recovering heavy oil is heating the reservoir to lower the viscosity of the heavy oil. Commonly used in-situ extraction thermal recovery techniques include a number of reservoir heating methods, such as steam flooding, cyclic steam stimulation, and steam assisted gravity drainage (SAGD). Steam flooding involves the use of injected steam to heat and physically displace hydrocarbons to encourage production of the hydrocarbons. Cyclic steam stimulation, also known as the huff and puff method, involves three stages, injection, soaking, and production. Steam is first injected into a well for a certain amount of time to heat the oil in the surrounding reservoir to a temperature at which it flows. After a sufficient injection of steam, the steam is usually left to “soak” for some time afterward (typically not more than a few days). Then oil may be produced out of the same well. Steam assisted gravity drainage, on the other hand, involves continuously injecting steam into an upper wellbore to heat the surrounding heavy crude oil and reduce its viscosity, causing the heated oil to drain into a lower wellbore, where it may be pumped out.
These conventional approaches are highly disadvantageous in that they are all significantly energy intensive. In some cases, these thermal recovery techniques are so inefficient that they are often non-economically viable for recovering heavy crude oil. Indeed, these conventional thermal recovery techniques are especially economically disadvantageous as applied to thin bitumen reservoirs (e.g. bitumen reservoirs having a thickness less than about 15 meters). The inefficiency of these various methods in thin bitumen reservoirs is in part due to the fact that heat applied to the reservoir can be lost to the over-burden or the under-burden, that is, geological material that lies above or below an area of economic interest. This heat loss to the over-burden and under-burden along with other factors such as well cost and well spacing makes conventional thermal recovery methods economically infeasible as applied to thin zone reservoirs.
Another conventional method for enhancing recovery of heavy crude oil is the use of solvents for dissolution to assist in the recovery of heavy crude oil. Unfortunately however, solvents can be quite expensive, and therefore, the process economics are highly sensitive to solvent cost and losses. Solvent losses increase in thin reservoirs, where solvent can become stranded due to reservoir heterogeneities and bypassed in the recovery process.
Accordingly, there is a need for enhanced recovery methods for heavy crude oil such as bitumen that address one or more of the disadvantages of the prior art.